1. Field of the Invention
This invention relates to the detection of electron (e)-beam scanning of a substrate.
2. Description of the Related Art
There are numerous instances in which an electrical circuit chip is scanned by an e-beam, generally with the use of a scanning electron microscope (SEM). E-beam scanning is used for trouble shooting problems with the circuitry, the injection of information at desired locations, voltage contrast analysis, quality control to determine whether the circuitry is functioning properly, and the measurement of line widths. The scanning is normally accomplished with a raster-type scan covering the full circuit area, although in certain instances the illumination of the substrate may be restricted to smaller designated portions of the circuitry.
One prior technique, referred to as "e-beam reconfiguration", uses an e-beam to reconfigure the circuit connections within an IC. This technique is described in Shaver, "Electron-Beam Customization, Repair, and Testing of the Wafer-Scale Circuits", Solid State Technology, February 1984, pages 135-139, and in Jensen et al., "Use of E-Beam for Random Access Read and Write of Digital Test Signals", 1984 IEEE Int'l. Solid-State Circuits Conf., pages 22-23, 310. It employs a number of redundant circuits that can be connected into or out of the main circuitry by flip-flop switches. Each flip-flop has two possible states, one of which connects a redundant circuit into the main circuitry and the other of which excludes the redundant circuit. A pair of e-beam sensitive diodes are provided for each flip-flop circuit, one to set the flip-flop in one state and the other to set the flip-flop in its other state. A small surface metallization is provided for each diode; directing an e-beam onto the metallization for a given diode produces a current flow through the diode and into the gate of one of the flip-flop transistors, causing the flip-flop to change state. The structure is designed so that it is necessary for the e-beam to dwell upon a surface diode metallization for a sufficient period of time to inject enough charge to actuate the flip-flop; this is not a problem because the locations of the diode metallizations are known, and the e-beam can be accurately directed to them.
In contrast to the e-beam reconfiguration technique in which the e-beam is held stationary at a known location on the substrate, it would be desirable to be able to detect when an e-beam is being scanned across the surface of an IC substrate. No systems are presently known that provide for the detection of a scanning e-beam. Furthermore, in some instances the upper surface of an IC substrate may be removed before an e-beam scan is performed, and in this event even the surface metallization used to capture electrons in the e-beam reconfiguration technique would no longer be available.